Conventionally, there has existed a developing device mainly incorporating a developing roller and a plurality of stirring and conveying screws. A developing device with this constitution produces the advantage of realization of size reduction. However, this developing device is inadequate in terms of uniform stirring of a developer, producing then such a disadvantage that it is inferior to ensure uniformity of image density in a copy page with solid black.
Namely, only when there is simply employed such a constitution in which one stirring and conveying screw feeds a developer to a developing roller and also collects the used developer and a plurality of stirring and conveying screws convey the developer in the axial direction each reversely, it is difficult to ensure density uniformity of the developer. When the mobility of the developer is degraded, the problem of occurrence of density non-uniformity corresponding to a screw pitch is produced in some cases. Especially in color equipment, copying of a high printing rate is frequently carried out. Therefore, it is critical to prevent image density non-uniformity.
To prevent such image density non-uniformity, a technology is disclosed in which a developer feed roller is provided as a transferring member of a developer from a developer stirring and conveying roller to a developer roller (for example, refer to Patent Document 1).
FIG. 6 shows a developing device 4 having the constitution disclosed in Patent Document 1. In FIG. 6, a developing roller 41, a first developer stirring and conveying roller 43, and a second developer stirring and conveying roller 44 are arranged in a lower housing 40A, and in the upper portion thereof, a developer feed roller 45 and a developer guide member 46 are arranged.
The developer feed roller 45 arranged above the first developer stirring and conveying roller 43 rotates a sleeve rotatably provided in the external circumference of a fixed magnet to feed a developer having been transferred from the first developer stirring and conveying roller 43 to the developing roller 41.
The developer conveyed from the flowing inlet 45A remains in a space 50A formed with the developing roller 41, a developer layer thickness regulating member 42, the developer feed roller 45, the developer guide member 46, and a space forming member 52, and a part thereof is fed to the developing roller 41 via the developer layer thickness regulating member 42.
The amount of the developer conveyed from the flowing inlet 45A is larger than that fed to the developing roller 41, whereby the amount of the developer remaining in the space (hereinafter also referred to as “developer standby space”) 50A is increased. Then, the overflowed developer falls down from a gap between the developer fed roller 45 and the developer guide member 46.
However, the mobility of the developer in the developing device varies in some cases, depending on conditions such as variation of the bulk state of the developer, variation of the ambience state, the durability of the developer, or downtime duration.
Especially when a large number of images are formed with relatively low printing rates, the consumption of a toner is decreased, whereby the toner remaining in the developing device for a long term is increased. Thereby, produced is such a problem that some additives coated on the external side of the toner particles penetrate into the interior of the toner via friction with a carrier, resulting in a decrease in developer mobility.
In the constitution described in Patent Document 1, when the mobility of a developer is decreased, in the case of a narrow gap between the developer feed roller 45 and the developer guide member 46, the developer is difficult to fall down from the gap, resulting in the possibility of an increase in the amount of the developer remaining in the developer standby space 50A. Further, when the gap is excessively widened, the amount falling down is increased and the amount remaining in the developer standby space 50A is decreased, whereby the amount of the developer feedable to the developing roller 41 is decreased, resulting in the problem that developer feeding to the developing roller 41 becomes unstable.
When the mobility of the developer remaining in the developer standby space 50A is decreased and then the amount of the developer is increased, the friction force between the toner and the carrier is increased, whereby an excessive stress is applied to the developer. When such an excessive stress is applied to the developer, the mobility of the toner is degraded due to burying of additives, and the charging performance of the carrier is deteriorated due to abrasion of the coated layer on the surface thereof, resulting in such a problem that image density non-uniformity tends to occur and then image quality is degraded. Further, due to a decrease in resistance via layer abrasion of the carrier, a phenomenon referred to as carrier development also tends to occur in which a carrier adheres to an image formed on an output sheet.
Further, due to an increase in the friction force between the toner and the carrier in the developing device, the temperature therein is increased, resulting in such a problem that the toner is easily softened. When the toner is softened, additives coated on the toner tend to penetrate into the interior of the toner, whereby there are produced such problems that adhesion force to the carrier is further increased and the softened toner is liable to adhere to various portions of the image forming apparatus.